1. Field of the Invention
The present invention relates to a process and an apparatus for treating a potassium chloride mixture with grains of different sizes, in particular with grains of up to 1.5 mm, in which the dry or moist potassium chloride is mashed with a potassium chloride solution and, in a continuous circulation, fed into a dissolving vessel.
2. The Prior Art
In Germany, crude salt obtained from potash deposits contains up to 75% sodium chloride and various secondary components such as magnesium and calcium in the form of chlorides, sulfates and bromides.
The quality standard of the potassium chloride produced therefrom has a content of valuable substances of about 60% to 62% K.sub.2 O, which conforms to a purity of 95% to 98% potassium chloride in the finished product.
In the extraction processes, traces of heavy metals (chromium, nickel, iron, molybdenum and vanadium) may get into the salt as well, conditioned by the materials of the plant equipment.
Since the impurities are present in different orders of magnitude, the problem of product refinement is manifold. An enhancement of the product primarily means increasing the overall purity, which has to be equated to a selective reduction of certain impurities.
Furthermore, for the use of the potassium chloride in practical life, the grain size or grain distribution plays an important role. If no "narrow grain cut" is desired exclusively, the "dust-free" product is desirable. Minimizing or eliminating the finest grains, which are responsible for the formation of dust, is one of the most important objectives.
In the discussion of processes already known for the production of potassium chloride products with higher purity, the fractionated crystallization of crude salt solutions is not addressed because a purity of more than 98% cannot be obtained with these known processes.
U.S. Pat. No. 4,385,902, which corresponds to German PS 2,852,925, proposes a process for the purification of potassium chloride crystals with a potassium chloride content of more than 97.5%, by extraction with a potassium chloride-saturated solution under isothermal conditions of 20.degree. C. to 70.degree. C. In this process, the sodium chloride content of the solution has to be under 35 g/L, preferably under 15 g/L. The treatment lasts 0.5 to 18 hours, whereby the rate of extraction process highly declines as the amounts of sodium chloride extracted increase. Seventy percent of the original sodium chloride content is extracted in the first four hours; the next 20% require a time expenditure of 12 hours. The rate of the process is highly dependent upon the size and structure of the grains and can be influenced only little by raising the temperature.
In addition to reducing the sodium chloride content as the main impurity, a reduction of the magnesium, calcium and bromine contents is found as well.
In German PS 3,129,042, an improvement of the process is achieved by using an extraction column operating countercurrently. In this case, sodium chloride concentrations in the treatment solution of up to 45 g/L are permissible; recommended is a concentration of less than 25 g/L. Another advantage is that it is possible also to purify coarser grains with a diameter of up to 4.7 mm.
German PS 4,014,370 specifies a digestion crystallization process in which small, contaminated crystals are flushed out in an aqueous medium together with larger, purer crystals. At the same time, a dissolution of the smaller crystals takes place, of which the order of magnitude is between 0.1 and 50 .mu.m, and a growth of the potassium chloride product crystals occurs. Since no evaporation is required in order to obtain the product potassium chloride, this process operates with a substantially lower expenditure of energy than the recrystallization processes. Even though a crystal with enlarged grains is obtained in this process, the product obtained remains finely granular, overall.
In U.S. Pat. No. 681,407, recrystallization is carried out by heating the suspension under pressure. The subsequent relief and introduction of new components such as, for example, magnesium ions, lead to salting out.
Based on the current state of knowledge and technology for the purification of potassium chloride with potassium chloride solutions, the following steps were carried out for enhancing or optimizing the procedure.
As opposed to German PS 3,129,042, where it is assumed that "the inorganic salt impurities are generally evenly distributed over the potassium chloride particles," one has to expect in the purification of potassium chloride crystals a non-uniform distribution of the impurities, depending on the grain size. The coarser crystals are contaminated more by inclusions of mother liquid. On the other hand, with the smaller grains, which have a larger specific surface, the adhering amount of secondary components present in the solution increases. From this follows that the grains of different grain sizes have to be treated differently in order to achieve a uniform reduction of the impurities.
The extraction of the foreign substances by treating the potassium chloride grains with a solution is a heterogeneous process. The lower chemical potential of the impurities in the solution is the driving force permitting the emigration of the foreign substances until an equilibrium is adjusted. This is a typical diffusion process in which the rate decreases as the process progresses because profiles of concentration develop from the interior of the grains. In order to achieve a uniform elimination of the impurities, coarser grains have to be extracted longer than the finer grains under the same conditions.
Microscopic and other investigations have shown that in the crystallization of potassium chloride from aqueous solutions, no monocrystals are formed, but rather aggregates and agglomerates whose original particles can have different dimensions. Therefore, a specific behavior has to be expected in the treatment of different products. This requires higher flexibility in the adjustment of the process conditions.
It is known from German PS 2,852,925 that temperature increases lead to a measurable acceleration of the process; however, various negative factors occur at the same time.
Due to the increase in concentration in the potassium chloride solution, the amount of potassium chloride per ton of treated product is increased. Furthermore, higher evaporation rates lead to undesirable potassium chloride deposits on the treated grains. A maximum temperature of 60.degree. C. to 70.degree. C. is justifiable under these conditions.
Reducing the distance for the diffusion process by crushing of the grains is permissible only in a limited way because this enlarges the specific surface of the grains, which leads to an increase of the adsorption capacity and accumulation of the impurities on the surface of the grains.
A "controlled dissolving" of about 10% of the grain volume with undersaturated potassium chloride solution, on the other hand, leads to a process acceleration of about one decimal power. The higher the degree of dissolution, the more impurities are removed from the volume of the grains per unit of time. Starting dissolution of the grains has to take place uniformly. Due to break-up of the closed pores, such dissolution leads to a loosening of the grain structure, so that the grains can be cleaned of adhering impurities in a superior way.